Targeting the Gastrointestinal Tract to Develop Novel Therapies for HIV RK Reeves1, A Burgener2,3,4 and NR Klatt5,6 Despite the use of antiretroviral therapy (ART), which delays and/or prevents AIDS pathogenesis, human immunodeficiency virus (HIV)infected individuals continue to face increased morbidities and mortality rates compared with uninfected individuals. Gastrointestinal (GI) mucosal dysfunction is a key feature of HIV infection, and is associated with mortality. In this study, we review current knowledge about mucosal dysfunction in HIV infection, and describe potential avenues for therapeutic targets to enhance mucosal function and decrease morbidities and mortalities in HIV-infected individuals. INTRODUCTION HIV infection in the ART era
With the recent optimization of antiretroviral drugs, most motivated human immunodeficiency virus (HIV)-infected patients with access to antiretroviral therapy (ART) can achieve durable and perhaps life-long viral suppression. Although these drugs improve quality of life, prevent AIDS, and reduce overall mortality, they do not fully restore health. A cure is not yet available, and, despite suppression of viremia with ART, individuals still have increased morbidity and mortality rates compared with uninfected persons.1–3 In addition, immune reconstitution with treatment is often incomplete even after many years of viral suppression.4 As compared to age-matched uninfected adults, ARTtreated HIV-infected adults have a higher risk of developing non-AIDS-related nonimmunological diseases, including cardiovascular disease, cancer, kidney disease, liver disease, neurologic disease, and bone disease.4,5 There are several factors that affect increased disease, such as CD4 count and viral load at onset of ART, immune reconstitution after ART, and mucosal dysfunction.4,5 Importantly, chronic mucosal dysfunction and lack of immune restoration in the gastrointestinal (GI) tract predicts and contributes to this excess risk of morbidity and mortality,1–3,6,7 characterized by epithelial barrier breaches, translocation of luminal microbes and microbial products, dysbiosis of the gut microbiome, and cellular immune dysfunction. In this review, we will describe mechanisms underlying mucosal dysfunction and potential avenues for novel therapeutic interventions, also summarized in Figure 1.
Barrier dysfunction and microbial factors
During HIV infection, focal breaches in the tight epithelial barrier of the GI tract occur, which allows microbes and microbialderived products to translocate across the barrier.8–10 This microbial translocation can directly induce inflammation by both direct stimulation of immune cells as well as by altering inflammatory pathways, such as tryptophan catabolism, and likely other consequences, such as hyperactivation and recruitment of innate immune cells. Furthermore, both microbial translocation1–3 and barrier damage6,7 have been found to be associated with morbidities and mortality in HIV infection. Although the exact mechanisms underlying barrier dysfunction are unclear, recent studies suggest that dysbiosis of the microbiome during HIV infection may play a role. Indeed, HIV infection induces alterations in the microbiome that leads to increased adherent and inflammatory microbes, such as Proteobacteria and Prevotella spp., along with loss of beneficial bacteria, such as Bacteroides spp.11–14 The result of this dysbiosis is increased inflammation, via mechanisms such as increased bacterial adherence to the epithelial barrier, increased tryptophan catabolism, and induction of inflammatory immune responses.11–14 However, the mechanisms underlying microbial dysbiosis in HIV infection have not been elucidated. The ability to improve mucosal dysfunction by targeting or preventing microbial translocation, dysbiosis of the microbiome, and/or barrier damage could potentially decrease morbidities and mortality in HIV infection. Microbial translocation has been targeted via therapeutics, such as sevelamer, which binds microbial lipopolysaccharide.15 Although this treatment in simian immunodeficiency virus (SIV)-infected macaques (a simian model of
1
Center for Virology and Vaccine Research, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts, USA; 2National Laboratory for HIV Immunology, Public Health Agency of Canada, Winnipeg, Canada; 3Department of Medical Microbiology, University of Manitoba, Winnipeg, Manitoba, Canada; 4Department of Medicine Solna, Center for Molecular Medicine, Karolinska Institute, Sweden; 5Department of Pharmaceutics, University of Washington, Seattle, Washington, USA; 6Washington National Primate Research Center, Seattle, Washington, USA. Correspondence: NR Klatt ([email protected]) Received 19 June 2015; accepted 10 July 2015; advance online publication 14 July 2015. doi:10.1002/cpt.186
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Figure 1 Potential targets for intervention to improve mucosal function in HIV infection.
HIV/AIDS) resulted in decreased microbial translocation and biomarkers of mortality, it was only demonstrated to be effective if given daily during acute infection.15 Given the difficulty in identifying individuals in acute infection, it is unlikely that sevelamer would be a reasonable therapeutic option if this is not shown to be effective in chronic, treated infection. Sevelamer was also given to HIV-infected individuals in a clinical trial, and although it did not show effects in depressing microbial translocation, potential cardiovascular benefits were observed.16 In another study, HIV-infected individuals with poor immune reconstitution were given hyperimmune bovine colostrum containing anti-lipopolysaccharide immunoglobulin.17 Although positive effects were observed on CD41 cell counts, there was no decrease in biomarkers of microbial translocation.17 Further studies with novel treatment strategies that may block lipopolysaccharide activity or enhance clearance of microbial products in circulation will be crucial. Targeting the microbiome directly may be an alternative approach to decrease mucosal damage and microbial translocation. One potential avenue is by enhancing the microbiome with probiotic (beneficial bacteria) treatment. A recent study found that supplementing ART with probiotics in chronically SIVinfected macaques resulted in enhanced GI tract CD41 T cell frequency and function, and decreased fibrosis, which could be highly beneficial to health in HIV-infected individuals.18 In addition, probiotics demonstrated decreased microbial translocation, which persisted after discontinuation in HIV-infected individuals.19 As such, probiotic therapy is beginning in clinical trials in 382
HIV-infected subjects through the AIDS clinical trials group. A more dramatic possibility of altering the microbiome is through fecal material transplantation (FMT). FMT is a procedure whereby full fecal material (typically from a screened donor) is transplanted in a patient through either an endoscope via the upper intestine or colonoscopy via the lower intestine. FMT is highly effective in improving the microbiome and curing disease associated with pathogenic bacteria, such as Clostridium difficile and vancomycin-resistant Enterococcus, as well inflammatory bowel disease, all of which cause chronic and severe diarrheal disease and have been effectively cured by FMT treatment.20,21 Given the extremely positive outcomes demonstrated in other GI ailments after FMT with limited side effects, this could be an ideal treatment option aimed at enhancing mucosal function in HIV-infected individuals. Finally, the epithelial barrier is likely damaged by inflammatory responses via activated immune cells in the GI tract. Below we will discuss cellular immunity and potential mechanisms and intervention strategies to enhance barrier function via altering mucosal immunity. Homeostatic cells in mucosal tissues
Multiple lymphocyte subpopulations contribute to regulation of the gut microenvironment and disruption of this delicate balance can result in gut barrier breakdown and subsequent microbial translocation. A prime axis of regulation is mediated by the overlapping functions of T helper cells that produce the homeostatic cytokines interleukin 17 (IL-17) or IL-22 (TH17 or TH22, VOLUME 98 NUMBER 4 | OCTOBER 2015 | www.wileyonlinelibrary/cpt
respectively) and the recently described, innate lymphoid cell type 3 (ILC3), all of which are particularly relevant to HIV because of their general restriction to mucosal sites.22–29 Through production of IL-17 and IL-22, these cells play critical roles in maintaining mucosal epithelial integrity, tissue remodeling and repair, and defense against intestinal pathogens. ILC3 have even been directly shown to promote anatomic containment of mucosal bacteria.30 During HIV disease and experimental SIV infection of macaques, all three cell types are depleted numerically, both by direct infection of T cells and inflammation-induced apoptosis, and potentially other unknown mechanisms, such as phenotype skewing. Depletion occurs early after infection, is maintained throughout disease, and only moderate cell recovery is observed after ART.31–33 In addition to cellular depletion, IL17 and IL-22 production are specifically suppressed and cells are skewed toward development of T regulatory cells or an inflammatory cytotoxic phenotype that is likely to further exacerbate disease.27,32–34 Indeed, loss of IL-17/22 production in SIV-infected macaques is directly associated with loss of epithelial integrity in the gut.34 Tryptophan catabolites, IL-2, IL-12, and IL-15, all of which are upregulated in the HIV/SIV-infected gut, antagonize RORgt and RORA (the transcription factors for TH17, TH22, and ILC3), promote cell apoptosis, and have all been directly implicated in depletion of these cells.27,32,33 The exquisite sensitivity of TH17, TH22, and ILC3 to changes in inflammatory mediators in the gut milieu suggests that targeting these HIV/ SIV-induced factors could be a meaningful approach to reverse cell depletion. Adiponectin and aldosterone therapy have both been used to increase total IL-17 and the frequency of TH17 cells.35,36 Ustekinumab (Stelara) and apilimod are potent IL-12 inhibitors used in patients with psoriasis,37 but also can inhibit IL-23, an important growth factor for TH17 and ILC3, suggesting combinatorial therapies could be a beneficial approach. JAK1 inhibitors have been shown to more specifically block IL-12 secretion from dendritic cells and could be a more directed target.38 Similarly, anti-IL-15 therapies have shown promise in treating autoimmune conditions and celiac disease,39,40 and small molecule inhibitors aimed at blocking the tryptophan catabolism pathway have also proven beneficial at decreasing inflammation and clinical outcomes in cancer therapy.41 Any combination of these therapies aimed at blocking HIV/SIV-induced inflammatory mediators could aid in restoring TH17, TH22, and ILC3 numbers and thus increase total gut IL-17 and IL-22 production and have a net stabilizing effect on the mucosal barrier. Inflammatory cells in mucosal tissues
Although local TH17, TH22, and ILC3 generally have a positive impact on maintaining gut homeostasis, HIV/SIV-induced inflammation can also attract hyperactivated immune cell populations that are normally less frequent or absent from the gut. Some notable innate effector cells that can be recruited to the gut during infection are natural killer (NK) cells, inflammatory dendritic cells, including plasmacytoid dendritic cells (pDCs) and myeloid dendritic cells (mDCs), and neutrophils. Under normal conditions, NK cells are relatively infrequent in the gut mucosa but are important for defense against various mucosal pathogens. However, in both CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 98 NUMBER 4 | OCTOBER 2015
HIV and SIV infection, the normal homing of NK cells in and out of lymph nodes is interrupted with an overall shift toward guttrafficking where they may accumulate in a hypercytotoxic state.27,42–46 Although increased NK cells could aid clearance of infected cells, overactivation can also result in nonspecific and bystander killing of immune cells.47–49 To this end, multiple strategies have been used to deplete NK cells in SIV-infected rhesus macaques, the most effective of which being JAK3 inhibitors.50–52 However, despite potential benefits to the gut, NK cell depletion in SIV-infected macaques has generally resulted in increased virus replication. Alternatively, NK cells observed in chronic HIV and SIV infection are significantly dysfunctional and may be unable to perform both regulatory functions and defense against opportunistic pathogens. Therefore, strategies not to deplete NK cells, but rather restore normal function may be a more appropriate strategy depending on clinical presentation. Altered dendritic cells in the gut could vastly affect overall immunity and regulation of the mucosal barrier given the important role of these cells in priming and inducing immunity. The pDCs are the primary IFN-a-producing cells in the body and, although they serve as a first line of defense against viral infection,53 accumulation of pDCs in the mucosa during HIV/SIV infection is associated with elevated levels of IFN-a in the gut, as well as increased immune activation, apoptosis, target cell recruitment, and dysregulation of anti-HIV/SIV T cell responses.54–58 Initial studies in acute SIV infection have shown that blocking IFN-a production by pDCs using TLR7/9 antagonists does not diminish immune activation, but this approach provided incomplete inhibition and was not specific to pDCs.59 Although specifically depleting pDCs in vivo is an attractive target, no effective approach has been identified outside of murine models. Alternatively, direct blocking of IFN-a with monoclonal antibody therapies, such as sifalimumab, have been effective in treatment of lupus, another disease with IFN-a-induced pathogenesis.60 AntiIFN-a vaccination has also been tried in patients with HIV, and, while generally well tolerated, the clinical benefits were very modest.61 Chloroquine has also been used to reduce IFN-a through its ability to inhibit endosomal acidification, and indeed these approaches significantly reduced immune activation and measures of microbial translocation.62 Collectively, these data suggest that, although pDCs and IFN-a might be beneficial if properly harnessed during transmission or acute disease,59 their overaccumulation in the gut during chronic infection is generally deleterious. As such, combination approaches to regulate pDC numbers and/or production of IFN-a in the gut could be attractive as a modality to improve disease outcomes. The mDCs are typically thought to be more important in priming and inducing antigen-specific immunity, particularly by presenting antigen to T cells. In HIV infection, dysbiosis of the microbiome has been demonstrated to result in altered mDCs, which in turn is associated with increased T cell activation both in mucosal and systemic sites.12,13 Although targeting mDCs could be dangerous in that reduction of function of these cells could result in decreased antigen-specific immunity, targeting the microbiome (as described above) may decrease the hyperactivation and alteration of mDCs in the GI tract and thus decrease inflammation. 383
Another potential consequence of dysbiosis of the microbiome is neutrophil accumulation in the GI tract, which has been demonstrated in HIV infected individuals.6 Given the propensity for neutrophils to produce high levels of inflammatory factors, such as proteases, which can directly cause breakdown of epithelial integrity, accumulation and hyperactivation of these cells because of microbial translocation and dysbiosis may be a direct source of mucosal damage. Neutrophils proteases, which include serine-type (cathepsin, elastase, proteinase 3) and matrix metalloproteinases,63 although secreted to combat invading microorganisms, can paradoxically damage host tissues by degrading structural proteins of mucosal surfaces.63,64 Serine proteases and matrix metalloproteinases cleave and redistribute cell–cell junction and adhesion proteins, including those of the EMC, and change endothelial and epithelial barrier integrity.65 Occludens, fibrins, collagen, cadherins, and others are all targets for matrix metalloproteinases, whereas neutrophil elastase targets EMC proteins, such as elastin and collagen.66 Antiproteases, such as serpins (serine protease inhibitors) and tissue inhibitor of metalloproteinases regulate their activity, but skewing of the protease-antiprotease balance is associated with barrier disruption, such as in asthma and chronic obstructive pulmonary disease,67 and the inability to clear bacterial infections and mucosal tissue damage.68 Sustained release of matrix metalloproteinases by accumulated neutrophils impedes repair of the intestinal epithelium by generation of cleavage products from surface of neutrophil-associated junctional adhesion proteins.69 In addition, a relationship between neutrophil proteases and mucosal barrier disruption in the female reproductive tract has been observed and proposed as a mechanism of increased HIV acquisition risk.70 Indeed, elevated serine antiproteases (serpins, elafin) are strongly linked with protection from HIV acquisition.71–74 Therefore, this demonstrates a link between the neutrophil proteases and mucosal barrier disruption in the context of HIV, and by extension these may serve as therapeutic potential for intervention in gut function in HIV infected individuals. CONCLUSION
Taken together, there are several potential avenues for therapeutic intervention in the GI tract during HIV infection that may benefit health (Figure 1). Currently, many studies are under way to address inflammation in HIV-infected individuals in the ART era, and, given the increased understanding of many mechanisms underlying GI dysfunction, microbial translocation and inflammation in the context of HIV infection has provided several novel pathways for intervention strategies. Ideally, supplementing ART with an anti-inflammatory or GI-enhancing regimen will be without side effects and simple to ensure patient adherence and outweigh the risks compared to the benefits. Overall, further understanding of the mechanisms underlying GI dysfunction and identifying specific targets for novel interventions should be highly prioritized to decrease morbidities and mortality in HIV infection. ACKNOWLEDGMENT The authors thank Laura Romas for her assistance in the creation of Figure 1. 384
CONFLICT OF INTEREST The authors declared no conflict of interest.
C 2015 American Society for Clinical Pharmacology and Therapeutics V
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With the recent optimization of antiretroviral drugs, most motivated human immunodeficiency virus (HIV)-infected patients with access to antiretroviral therapy (ART) can achieve durable and perhaps life-long viral suppression. Although these drugs improve quality of life, prevent AIDS, and reduce overall mortality, they do not fully restore health. A cure is not yet available, and, despite suppression of viremia with ART, individuals still have increased morbidity and mortality rates compared with uninfected persons.1–3 In addition, immune reconstitution with treatment is often incomplete even after many years of viral suppression.4 As compared to age-matched uninfected adults, ARTtreated HIV-infected adults have a higher risk of developing non-AIDS-related nonimmunological diseases, including cardiovascular disease, cancer, kidney disease, liver disease, neurologic disease, and bone disease.4,5 There are several factors that affect increased disease, such as CD4 count and viral load at onset of ART, immune reconstitution after ART, and mucosal dysfunction.4,5 Importantly, chronic mucosal dysfunction and lack of immune restoration in the gastrointestinal (GI) tract predicts and contributes to this excess risk of morbidity and mortality,1–3,6,7 characterized by epithelial barrier breaches, translocation of luminal microbes and microbial products, dysbiosis of the gut microbiome, and cellular immune dysfunction. In this review, we will describe mechanisms underlying mucosal dysfunction and potential avenues for novel therapeutic interventions, also summarized in Figure 1.
Barrier dysfunction and microbial factors
During HIV infection, focal breaches in the tight epithelial barrier of the GI tract occur, which allows microbes and microbialderived products to translocate across the barrier.8–10 This microbial translocation can directly induce inflammation by both direct stimulation of immune cells as well as by altering inflammatory pathways, such as tryptophan catabolism, and likely other consequences, such as hyperactivation and recruitment of innate immune cells. Furthermore, both microbial translocation1–3 and barrier damage6,7 have been found to be associated with morbidities and mortality in HIV infection. Although the exact mechanisms underlying barrier dysfunction are unclear, recent studies suggest that dysbiosis of the microbiome during HIV infection may play a role. Indeed, HIV infection induces alterations in the microbiome that leads to increased adherent and inflammatory microbes, such as Proteobacteria and Prevotella spp., along with loss of beneficial bacteria, such as Bacteroides spp.11–14 The result of this dysbiosis is increased inflammation, via mechanisms such as increased bacterial adherence to the epithelial barrier, increased tryptophan catabolism, and induction of inflammatory immune responses.11–14 However, the mechanisms underlying microbial dysbiosis in HIV infection have not been elucidated. The ability to improve mucosal dysfunction by targeting or preventing microbial translocation, dysbiosis of the microbiome, and/or barrier damage could potentially decrease morbidities and mortality in HIV infection. Microbial translocation has been targeted via therapeutics, such as sevelamer, which binds microbial lipopolysaccharide.15 Although this treatment in simian immunodeficiency virus (SIV)-infected macaques (a simian model of
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Center for Virology and Vaccine Research, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts, USA; 2National Laboratory for HIV Immunology, Public Health Agency of Canada, Winnipeg, Canada; 3Department of Medical Microbiology, University of Manitoba, Winnipeg, Manitoba, Canada; 4Department of Medicine Solna, Center for Molecular Medicine, Karolinska Institute, Sweden; 5Department of Pharmaceutics, University of Washington, Seattle, Washington, USA; 6Washington National Primate Research Center, Seattle, Washington, USA. Correspondence: NR Klatt ([email protected]) Received 19 June 2015; accepted 10 July 2015; advance online publication 14 July 2015. doi:10.1002/cpt.186
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Figure 1 Potential targets for intervention to improve mucosal function in HIV infection.
HIV/AIDS) resulted in decreased microbial translocation and biomarkers of mortality, it was only demonstrated to be effective if given daily during acute infection.15 Given the difficulty in identifying individuals in acute infection, it is unlikely that sevelamer would be a reasonable therapeutic option if this is not shown to be effective in chronic, treated infection. Sevelamer was also given to HIV-infected individuals in a clinical trial, and although it did not show effects in depressing microbial translocation, potential cardiovascular benefits were observed.16 In another study, HIV-infected individuals with poor immune reconstitution were given hyperimmune bovine colostrum containing anti-lipopolysaccharide immunoglobulin.17 Although positive effects were observed on CD41 cell counts, there was no decrease in biomarkers of microbial translocation.17 Further studies with novel treatment strategies that may block lipopolysaccharide activity or enhance clearance of microbial products in circulation will be crucial. Targeting the microbiome directly may be an alternative approach to decrease mucosal damage and microbial translocation. One potential avenue is by enhancing the microbiome with probiotic (beneficial bacteria) treatment. A recent study found that supplementing ART with probiotics in chronically SIVinfected macaques resulted in enhanced GI tract CD41 T cell frequency and function, and decreased fibrosis, which could be highly beneficial to health in HIV-infected individuals.18 In addition, probiotics demonstrated decreased microbial translocation, which persisted after discontinuation in HIV-infected individuals.19 As such, probiotic therapy is beginning in clinical trials in 382
HIV-infected subjects through the AIDS clinical trials group. A more dramatic possibility of altering the microbiome is through fecal material transplantation (FMT). FMT is a procedure whereby full fecal material (typically from a screened donor) is transplanted in a patient through either an endoscope via the upper intestine or colonoscopy via the lower intestine. FMT is highly effective in improving the microbiome and curing disease associated with pathogenic bacteria, such as Clostridium difficile and vancomycin-resistant Enterococcus, as well inflammatory bowel disease, all of which cause chronic and severe diarrheal disease and have been effectively cured by FMT treatment.20,21 Given the extremely positive outcomes demonstrated in other GI ailments after FMT with limited side effects, this could be an ideal treatment option aimed at enhancing mucosal function in HIV-infected individuals. Finally, the epithelial barrier is likely damaged by inflammatory responses via activated immune cells in the GI tract. Below we will discuss cellular immunity and potential mechanisms and intervention strategies to enhance barrier function via altering mucosal immunity. Homeostatic cells in mucosal tissues
Multiple lymphocyte subpopulations contribute to regulation of the gut microenvironment and disruption of this delicate balance can result in gut barrier breakdown and subsequent microbial translocation. A prime axis of regulation is mediated by the overlapping functions of T helper cells that produce the homeostatic cytokines interleukin 17 (IL-17) or IL-22 (TH17 or TH22, VOLUME 98 NUMBER 4 | OCTOBER 2015 | www.wileyonlinelibrary/cpt
respectively) and the recently described, innate lymphoid cell type 3 (ILC3), all of which are particularly relevant to HIV because of their general restriction to mucosal sites.22–29 Through production of IL-17 and IL-22, these cells play critical roles in maintaining mucosal epithelial integrity, tissue remodeling and repair, and defense against intestinal pathogens. ILC3 have even been directly shown to promote anatomic containment of mucosal bacteria.30 During HIV disease and experimental SIV infection of macaques, all three cell types are depleted numerically, both by direct infection of T cells and inflammation-induced apoptosis, and potentially other unknown mechanisms, such as phenotype skewing. Depletion occurs early after infection, is maintained throughout disease, and only moderate cell recovery is observed after ART.31–33 In addition to cellular depletion, IL17 and IL-22 production are specifically suppressed and cells are skewed toward development of T regulatory cells or an inflammatory cytotoxic phenotype that is likely to further exacerbate disease.27,32–34 Indeed, loss of IL-17/22 production in SIV-infected macaques is directly associated with loss of epithelial integrity in the gut.34 Tryptophan catabolites, IL-2, IL-12, and IL-15, all of which are upregulated in the HIV/SIV-infected gut, antagonize RORgt and RORA (the transcription factors for TH17, TH22, and ILC3), promote cell apoptosis, and have all been directly implicated in depletion of these cells.27,32,33 The exquisite sensitivity of TH17, TH22, and ILC3 to changes in inflammatory mediators in the gut milieu suggests that targeting these HIV/ SIV-induced factors could be a meaningful approach to reverse cell depletion. Adiponectin and aldosterone therapy have both been used to increase total IL-17 and the frequency of TH17 cells.35,36 Ustekinumab (Stelara) and apilimod are potent IL-12 inhibitors used in patients with psoriasis,37 but also can inhibit IL-23, an important growth factor for TH17 and ILC3, suggesting combinatorial therapies could be a beneficial approach. JAK1 inhibitors have been shown to more specifically block IL-12 secretion from dendritic cells and could be a more directed target.38 Similarly, anti-IL-15 therapies have shown promise in treating autoimmune conditions and celiac disease,39,40 and small molecule inhibitors aimed at blocking the tryptophan catabolism pathway have also proven beneficial at decreasing inflammation and clinical outcomes in cancer therapy.41 Any combination of these therapies aimed at blocking HIV/SIV-induced inflammatory mediators could aid in restoring TH17, TH22, and ILC3 numbers and thus increase total gut IL-17 and IL-22 production and have a net stabilizing effect on the mucosal barrier. Inflammatory cells in mucosal tissues
Although local TH17, TH22, and ILC3 generally have a positive impact on maintaining gut homeostasis, HIV/SIV-induced inflammation can also attract hyperactivated immune cell populations that are normally less frequent or absent from the gut. Some notable innate effector cells that can be recruited to the gut during infection are natural killer (NK) cells, inflammatory dendritic cells, including plasmacytoid dendritic cells (pDCs) and myeloid dendritic cells (mDCs), and neutrophils. Under normal conditions, NK cells are relatively infrequent in the gut mucosa but are important for defense against various mucosal pathogens. However, in both CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 98 NUMBER 4 | OCTOBER 2015
HIV and SIV infection, the normal homing of NK cells in and out of lymph nodes is interrupted with an overall shift toward guttrafficking where they may accumulate in a hypercytotoxic state.27,42–46 Although increased NK cells could aid clearance of infected cells, overactivation can also result in nonspecific and bystander killing of immune cells.47–49 To this end, multiple strategies have been used to deplete NK cells in SIV-infected rhesus macaques, the most effective of which being JAK3 inhibitors.50–52 However, despite potential benefits to the gut, NK cell depletion in SIV-infected macaques has generally resulted in increased virus replication. Alternatively, NK cells observed in chronic HIV and SIV infection are significantly dysfunctional and may be unable to perform both regulatory functions and defense against opportunistic pathogens. Therefore, strategies not to deplete NK cells, but rather restore normal function may be a more appropriate strategy depending on clinical presentation. Altered dendritic cells in the gut could vastly affect overall immunity and regulation of the mucosal barrier given the important role of these cells in priming and inducing immunity. The pDCs are the primary IFN-a-producing cells in the body and, although they serve as a first line of defense against viral infection,53 accumulation of pDCs in the mucosa during HIV/SIV infection is associated with elevated levels of IFN-a in the gut, as well as increased immune activation, apoptosis, target cell recruitment, and dysregulation of anti-HIV/SIV T cell responses.54–58 Initial studies in acute SIV infection have shown that blocking IFN-a production by pDCs using TLR7/9 antagonists does not diminish immune activation, but this approach provided incomplete inhibition and was not specific to pDCs.59 Although specifically depleting pDCs in vivo is an attractive target, no effective approach has been identified outside of murine models. Alternatively, direct blocking of IFN-a with monoclonal antibody therapies, such as sifalimumab, have been effective in treatment of lupus, another disease with IFN-a-induced pathogenesis.60 AntiIFN-a vaccination has also been tried in patients with HIV, and, while generally well tolerated, the clinical benefits were very modest.61 Chloroquine has also been used to reduce IFN-a through its ability to inhibit endosomal acidification, and indeed these approaches significantly reduced immune activation and measures of microbial translocation.62 Collectively, these data suggest that, although pDCs and IFN-a might be beneficial if properly harnessed during transmission or acute disease,59 their overaccumulation in the gut during chronic infection is generally deleterious. As such, combination approaches to regulate pDC numbers and/or production of IFN-a in the gut could be attractive as a modality to improve disease outcomes. The mDCs are typically thought to be more important in priming and inducing antigen-specific immunity, particularly by presenting antigen to T cells. In HIV infection, dysbiosis of the microbiome has been demonstrated to result in altered mDCs, which in turn is associated with increased T cell activation both in mucosal and systemic sites.12,13 Although targeting mDCs could be dangerous in that reduction of function of these cells could result in decreased antigen-specific immunity, targeting the microbiome (as described above) may decrease the hyperactivation and alteration of mDCs in the GI tract and thus decrease inflammation. 383
Another potential consequence of dysbiosis of the microbiome is neutrophil accumulation in the GI tract, which has been demonstrated in HIV infected individuals.6 Given the propensity for neutrophils to produce high levels of inflammatory factors, such as proteases, which can directly cause breakdown of epithelial integrity, accumulation and hyperactivation of these cells because of microbial translocation and dysbiosis may be a direct source of mucosal damage. Neutrophils proteases, which include serine-type (cathepsin, elastase, proteinase 3) and matrix metalloproteinases,63 although secreted to combat invading microorganisms, can paradoxically damage host tissues by degrading structural proteins of mucosal surfaces.63,64 Serine proteases and matrix metalloproteinases cleave and redistribute cell–cell junction and adhesion proteins, including those of the EMC, and change endothelial and epithelial barrier integrity.65 Occludens, fibrins, collagen, cadherins, and others are all targets for matrix metalloproteinases, whereas neutrophil elastase targets EMC proteins, such as elastin and collagen.66 Antiproteases, such as serpins (serine protease inhibitors) and tissue inhibitor of metalloproteinases regulate their activity, but skewing of the protease-antiprotease balance is associated with barrier disruption, such as in asthma and chronic obstructive pulmonary disease,67 and the inability to clear bacterial infections and mucosal tissue damage.68 Sustained release of matrix metalloproteinases by accumulated neutrophils impedes repair of the intestinal epithelium by generation of cleavage products from surface of neutrophil-associated junctional adhesion proteins.69 In addition, a relationship between neutrophil proteases and mucosal barrier disruption in the female reproductive tract has been observed and proposed as a mechanism of increased HIV acquisition risk.70 Indeed, elevated serine antiproteases (serpins, elafin) are strongly linked with protection from HIV acquisition.71–74 Therefore, this demonstrates a link between the neutrophil proteases and mucosal barrier disruption in the context of HIV, and by extension these may serve as therapeutic potential for intervention in gut function in HIV infected individuals. CONCLUSION
Taken together, there are several potential avenues for therapeutic intervention in the GI tract during HIV infection that may benefit health (Figure 1). Currently, many studies are under way to address inflammation in HIV-infected individuals in the ART era, and, given the increased understanding of many mechanisms underlying GI dysfunction, microbial translocation and inflammation in the context of HIV infection has provided several novel pathways for intervention strategies. Ideally, supplementing ART with an anti-inflammatory or GI-enhancing regimen will be without side effects and simple to ensure patient adherence and outweigh the risks compared to the benefits. Overall, further understanding of the mechanisms underlying GI dysfunction and identifying specific targets for novel interventions should be highly prioritized to decrease morbidities and mortality in HIV infection. ACKNOWLEDGMENT The authors thank Laura Romas for her assistance in the creation of Figure 1. 384
CONFLICT OF INTEREST The authors declared no conflict of interest.
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